Test for modifier keys correctly

In a GdkEventKey, the
state field is a bit mask which
indicates the modifier state at the time the key was pressed.
Modifiers are keys like Control and
NumLock. When implementing a
“key-press-event” handler, you should use
gtk_accelerator_get_default_mod_mask() to
test against modifier keys. This function returns a bit mask
which encompasses all the modifiers which the user may be
actively pressing, such as Control,
Shift, and Alt, but ignores
"innocuous" modifiers such as NumLock and
CapsLock.

Say you want to see if
Control+F10
was pressed. Doing a simple test like
event->keysym == GDK_F10 &&
event->state == GDK_CONTROL_MASK is not
enough. If CapsLock is pressed, then
event->state will be equal to
GDK_CONTROL_MASK | GDK_LOCK_MASK, and the
simple test will fail. By taking the logical-and of
event->state and
gtk_accelerator_get_default_mod_mask(), you
can ignore the modifiers which are not actively pressed by the
user at the same time as the base key.